Apparatus for fabricating compound semiconductor device

ABSTRACT

A system for fabricating a compound semiconductor device includes a gas treatment apparatus that performs a hydrogen chloride gas etching on a compound semiconductor substrate, a radical treatment apparatus that performs a radical hydrotreatment on the substrate, a semiconductor film forming apparatus that forms a compound semiconductor film on the treated substrate, a conductive film forming apparatus that forms a conductive film on the substrate, and an ultrahigh vacuum transfer path that connects together the several apparatuses so that the substrate being processed can be transferred through the transfer path from apparatus to apparatus under a continuously maintained ultrahigh vacuum environment. Some of the apparatuses can overlap or share functions with one another.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application is a Divisional Application of copendingU.S. patent application Ser. No. 09/818,309, filed on Mar. 27, 2001.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a method of fabricating acompound semiconductor device employed for an LED (light emitting diode)or the like and an apparatus for fabricating a compound semiconductordevice, and more particularly, it relates to a method of fabricating asemiconductor device for a ZnSe-based LED employed for the backlight ofa liquid display unit or the like and an apparatus for fabricating acompound semiconductor device.

[0004] 2. Description of the Prior Art

[0005] In order to drive a compound semiconductor device such as an LED,it is necessary to form an electrode on a compound semiconductor. Forexample, an electrode of ohmic contact must be formed on the backsurface of a ZnSe substrate, in order to produce a ZnSe-based LED chip.However, such an electrode of ohmic contact cannot be readily formed onthe ZnSe substrate for the following reasons:

[0006] (a) The upper bound of the carrier concentration of the ZnSesubstrate is limited to the latter half of the 10¹⁷ mark. No ZnSesubstrate having a carrier concentration exceeding the latter half ofthe 10¹⁷ mark has heretofore been fabricated. In particular, the carrierconcentration of a p-type ZnSe substrate cannot even reach the 10¹⁷mark.

[0007] (b) Oxides are readily formed on the surface of the substrate.

[0008] (c) The treatment temperature for forming the electrode must benot more than 250° C., in order to protect an active layer or a claddinglayer of an emission part.

[0009] In general, In which is a low melting point metal is known as anelectrode metal implementing ohmic contact. An electrode of ohmiccontact can be formed also on the aforementioned ZnSe substrate byfusing In.

[0010] In a ZnSe-based LED prepared by fusing In, however, variousinconveniences result from the low melting point of In in solder reflowor transfer molding. When the ZnSe-based LED is heated to 200° C. to250° C., for example, ball-up results from the low melting point of Inof about 157° C., and hence a flat interface cannot be obtained.Therefore, uniform ohmic contact cannot be attained despiteimplementation of partial ohmic contact. When a flat electrode havingohmic contact is not formed, an unnecessarily high voltage must beapplied to the overall LED, which in turn requires a large number ofbatteries and cannot be readily applied to the backlight for a liquidcrystal display screen of a portable telephone or the like. Thus,strongly awaited is development of an electrode, not prepared from In,capable of attaining thermally and mechanically stable ohmic contactwith a compound semiconductor.

SUMMARY OF THE INVENTION

[0011] An object of the present invention is to provide a method offabricating a compound semiconductor device having an electrodeattaining stable ohmic contact with a compound semiconductor withoutemploying a low melting point metal such as In and an apparatus forfabricating a compound semiconductor device.

[0012] The method of fabricating a compound semiconductor deviceaccording to the present invention comprises a substrate cleaning stepincluding a first cleaning step of heating a compound semiconductorsubstrate containing a first conductivity type impurity in a temperaturerange of not more than 250° C. for etching its surface with hydrogenchloride and a second cleaning step of performing a radicalhydrotreatment on the compound semiconductor substrate etched withhydrogen chloride after the first cleaning step.

[0013] When etching the surface of the compound semiconductor substratewith hydrogen chloride, an oxide film resulting from atmosphericexposure can be removed. In this hydrogen chloride etching, thetemperature of the compound semiconductor substrate must be not morethan 250° C., in order to prevent damage of an active layer and acladding layer already formed on the opposite surface of the compoundsemiconductor substrate. When performing the hydrogen chloride treatmentat a temperature of not more than 250° C., the surface of the compoundsemiconductor substrate adsorbs Cl although the oxide film or a carbidecan be removed from this surface. Therefore, the radical hydrotreatmentis performed for removing the adsorbed Cl. Cl can be removed by theradical hydrotreatment. Radical hydrogen has high reaction activity, andhence a sufficiently high radical hydrogenation reaction rate can beensured also when setting the temperature of the compound semiconductorsubstrate to not more than 250° C. A clean surface of a compoundsemiconductor can be obtained also at a temperature of not more than250° C. due to the hydrogen chloride treatment and the radicalhydrotreatment, so that an epitaxial compound semiconductor film can beformed without forming interfacial levels. The compound semiconductorsubstrate includes not only the bare substrate in the initial stage ofthe treatments but also the compound semiconductor substrate formed witha thin film such as an optical active layer in the process of thetreatments. The present invention is mainly directed to a case offorming an ohmic electrode layer on the back surface of a compoundsemiconductor substrate already formed with an optical active layer orthe like on its surface without damaging the optical active layer or thelike.

[0014] The method of fabricating a compound semiconductor deviceaccording to the present invention can further comprise a compoundsemiconductor film forming step of epitaxially growing a compoundsemiconductor film containing the first conductivity type impurity in ahigher concentration than the compound semiconductor substrate on thecompound semiconductor substrate after the cleaning step and aconductive electrode film forming step of forming a conductive electrodefilm on the compound semiconductor film.

[0015] When directly forming a conductive electrode layer of a materialother than In on the compound semiconductor substrate, ohmic contactcannot be attained if the compound semiconductor substrate has a lowcarrier concentration. Therefore, the compound semiconductor filmcontaining a conductive impurity in a higher concentration than thecompound semiconductor substrate is epitaxially grown for forming theconductive electrode film on the epitaxial film having a high carrierconcentration and ensuring ohmic contact. If merely performing cleaningby general etching after exposing the surface of the compoundsemiconductor substrate to the atmosphere, a desired compoundsemiconductor film cannot be obtained due to a large quantity ofimpurities remaining on the surface. Therefore, when performing filmformation at a low temperature of not more than 250° C., for example, anepitaxial film having a high carrier concentration cannot be obtainedwith a small number of interfacial levels on the surface of the compoundsemiconductor substrate.

[0016] If the interface between the compound semiconductor film and theconductive electrode film is not clean, large interfacial resistance isformed to cause such large potential difference on the interface thatohmic contact is not implemented, or voltage applied to the overall LEDcannot be reduced even if ohmic contact is implemented. According to theaforementioned structure of the present invention, a highly cleansurface can be obtained on the back side of the compound semiconductorsubstrate without damaging an optical active layer or the like alreadyformed on its surface, for readily forming an electrode of ohmic contactthereon.

[0017] An active layer and a cladding layer serving as emission partsare formed on another surface of an n-type ZnSe substrate. The activelayer and the cladding layer stably emit blue light, while the ZnSesubstrate receiving this blue light emits yellow light. Therefore,highly stable white light can be obtained through the low-pricedcompound semiconductor device. According to the present invention, theelectrode of ohmic contact is provided on the back surface of theaforementioned n-type ZnSe substrate for reducing a contact potential,thereby enabling reduction of the number of batteries necessary for thebacklight of a liquid crystal display unit for a portable terminal orthe like.

[0018] The aforementioned method of fabricating a compound semiconductordevice according to the present invention can carry out treatments inthe compound semiconductor film forming step and the conductiveelectrode film forming step within such a temperature range that thetemperature of the compound semiconductor substrate is not more than250° C.

[0019] Thus, the electrode layer of ohmic contact can be readily formedon the back surface of the compound semiconductor substrate withoutdamaging an active layer or the like already formed on the surface ofthe compound semiconductor substrate.

[0020] In the aforementioned method of fabricating a compoundsemiconductor device according to the present invention, treatments inthe substrate cleaning step, the compound semiconductor film formingstep and the conductive electrode film forming step are preferablycontinuously carried out without exposing the compound semiconductorsubstrate to the atmosphere, for example.

[0021] If both of the interface between the compound semiconductorsubstrate and the compound semiconductor film and that between thecompound semiconductor film and the conductive electrode film are notcleaned, large interfacial resistance is formed to cause large potentialdifference on the interfaces. If the interface between the compoundsemiconductor substrate and the compound semiconductor film is exposedto the atmosphere, the effects of the hydrogen chloride treatment andthe radical hydrotreatment are reduced by half. The interface betweenthe compound semiconductor film and the conductive electrode film tendsto be exposed to the atmosphere. When exposed to the atmosphere, theinterface is contaminated with oxides or carbon and hence ohmic contactis hard to implement or voltage applied to the overall LED cannot bereduced even if ohmic contact is implemented. When the compoundsemiconductor device is used as the backlight of a liquid crystaldisplay unit for a portable terminal, therefore, the number of necessarybatteries cannot be reduced. Ohmic contact of low resistance can beimplemented by continuously forming the compound semiconductor film andthe conductive electrode film on the clean surface of the substrate andthe compound semiconductor film respectively without atmosphericexposure, as described above.

[0022] In the aforementioned method of fabricating a compoundsemiconductor device according to the present invention, treatments inthe substrate cleaning step, the compound semiconductor film formingstep and the conductive electrode film forming step are preferablycarried out in respective treatment chambers of a treatment apparatushaving a plurality of treatment chambers coupled with each other by anultrahigh vacuum transfer path respectively without exposing thecompound semiconductor substrate to the atmosphere between thetreatments, for example.

[0023] The compound semiconductor substrate can be heated to atemperature of not more than 250° C. not damaging the optical activelayer or the like without atmospheric exposure, for obtaining a cleansurface in an assembly-line manner and forming the electrode film ofohmic contact. Thus, the electrode film of ohmic contact can beextremely readily formed on the back surface of the compoundsemiconductor substrate, for improving the fabrication yield of an LEDemitting white light, for example, and reducing the fabrication cost.

[0024] In the aforementioned method of fabricating a compoundsemiconductor device according to the present invention, the firstcleaning step can be carried out under conditions of (a) gas components5 to 20 volume % of HCl and a rest of He gas, and (b) gas pressure of1×10⁻⁶ Torr to 1×10⁻⁴ Torr, for example.

[0025] The surface of the compound semiconductor substrate is positivelyetched due to the aforementioned hydrogen chloride cleaning, so thatoxides are removed. Thus, one condition for epitaxially growing ahigh-concentration carrier film is satisfied. In relation to the gascomposition with the rest of He gas, oxides are insufficiently removedif the content of HCl is less than 5 volume %, while the etched surfaceis not flattened if the content exceeds 20 volume %. Further, theetching rate is so small that etching is practically impossible if thetotal gas pressure is less than 1×10⁻⁶ Torr in the aforementioned gascomposition, while etching is ununiformly performed to deterioratesurface flatness if the gas pressure exceeds 1×10⁻⁴ Torr. If thesubstrate temperature exceeds 250° C., further, the performance of aportion formed as the active layer etc. of the LED is damaged.

[0026] In the aforementioned method of fabricating a compoundsemiconductor device according to the present invention, hydrogenradicalization in the second cleaning step can be carried out underconditions of (a) hydrogen pressure of 5×10⁻⁷ Torr to 5×10⁻⁴ Torr, and(b) radicalization power of 50 to 300 W, for example.

[0027] Adsorbed Cl can be removed for obtaining a clean surface by theradical hydrotreatment carried out at a temperature of not more than250° C. under the aforementioned conditions.

[0028] In the aforementioned method of fabricating a compoundsemiconductor device according to the present invention, the compoundsemiconductor substrate can be an n-type ZnSe substrate containing ann-type impurity of at least 1×10¹⁷/cm³, for example, the compoundsemiconductor film can be an n⁺-type ZnSe film containing an n-typeimpurity of at least 1×10¹⁹/cm³, and the conductive electrode film caninclude a Ti film in contact with the n⁺-type ZnSe film and a protectivefilm protecting the Ti film.

[0029] An active layer and a cladding layer serving as emission partsare formed on the other surface of the n-type ZnSe substrate. The activelayer etc. stably emit blue light, while the ZnSe substrate receivingthis blue light emits yellow light. Therefore, highly stable white lightcan be obtained through the low-priced compound semiconductor device.The electrode of ohmic contact according to the present invention isprovided on the back surface of the aforementioned n-type ZnSe substratefor reducing a contact potential, thereby enabling reduction of thenumber of batteries necessary for the backlight of a liquid crystaldisplay unit for a portable terminal or the like. Electric resistancecan be reduced by setting the n-type impurity concentration in then-type ZnSe substrate to at least 1×10¹⁷/cm³, while ohmic contact can beimplemented by setting the n-type impurity concentration of the n⁺-typeZnSe film to at least 1×10¹⁹/cm³. If the n-type impurity concentrationof the n⁺-type ZnSe film is less than 1×10¹⁹/cm³, ohmic contact cannotbe attained between this film and a conductive layer such as the Ti filmbut interfacial resistance is disadvantageously increased. If the n-typeimpurity concentration in the n-type ZnSe substrate is less than1×10¹⁷/cm³, electric resistance is disadvantageously increased. Suchincrease of the resistance increases the number of necessary batteries,to increase the weight of and the cost for the compound semiconductordevice.

[0030] Ti readily implements ohmic contact with respect to the compoundsemiconductor film containing the aforementioned first conductivity typeimpurity, such as an n-type impurity, for example, in a highconcentration. Therefore, the Ti film is formed to implement ohmiccontact, and a chemically stable Au film, for example, is formed on theactive Ti film.

[0031] The apparatus for fabricating a compound semiconductor deviceaccording to the present invention comprises a gas treatment apparatusperforming gas etching on a compound semiconductor substrate, a radicaltreatment chamber performing a radical treatment on the compoundsemiconductor substrate, a film formation treatment apparatus forming acompound semiconductor film on the compound semiconductor substrate anda conductive film forming apparatus forming a conductive film on thecompound semiconductor substrate, and further comprises an ultrahighvacuum transfer path coupled with the respective ones of the gastreatment apparatus, the radical treatment apparatus, the film formationtreatment apparatus and the conductive film forming apparatus to becapable of transferring the compound semiconductor substrate.

[0032] It is possible to continuously perform a hydrogen chloridetreatment and a radical hydrotreatment in an ultrahigh vacuum, reduceinterfacial resistance and form an epitaxial compound semiconductor filmby employing the treatment apparatus having the aforementionedstructure. The ultrahigh vacuum is a vacuum of less than 10⁻⁸ Torr. Thecompound semiconductor device according to the present invention can befabricated only by the fabrication apparatus having the aforementionedstructure.

[0033] In the aforementioned apparatus for fabricating a compoundsemiconductor device according to the present invention, the gastreatment apparatus can include a hydrogen chloride treatment apparatusetching the compound semiconductor substrate with hydrogen chloride, forexample.

[0034] Oxygen and carbon adhering to the surface of the compoundsemiconductor substrate can be removed by positively etching thecompound semiconductor substrate with the hydrogen chloride treatmentapparatus connected by the ultrahigh vacuum transfer path, for movingthe compound semiconductor substrate to a next step in the ultrahighvacuum.

[0035] In the aforementioned apparatus for fabricating a compoundsemiconductor device according to the present invention, the radicaltreatment apparatus can include a radical hydrotreatment apparatusperforming a radical hydrotreatment on the compound semiconductorsubstrate, for example.

[0036] When performing the hydrogen chloride treatment at a temperatureof not more than 250° C., Cl is absorbed on the compound semiconductorsubstrate to form interfacial levels when the compound semiconductorfilm is formed on the compound semiconductor substrate. Therefore, theradical hydrotreatment is carried out after the hydrogen chloridetreatment while moving the compound semiconductor in the ultrahighvacuum without atmospheric exposure, thereby removing Cl so that a cleansurface appears and an epitaxial film can be formed without interfaciallevels.

[0037] The aforementioned apparatus for fabricating a compoundsemiconductor device according to the present invention can have an MBE(molecular beam epitaxial) apparatus serving both as the radicalhydrotreatment apparatus and the film formation treatment apparatus, forexample, and the MBE apparatus can include a Zn cell, an Se cell, aZnCl₂ cell, a hydrogen gas supply source and a radicalization gun forperforming a radical hydrotreatment on the compound semiconductorsubstrate with the hydrogen gas supply source and the radicalization gunand forming an n-type ZnSe film on the compound semiconductor substratewith the Zn cell, the Se cell and the ZnCl₂ cell.

[0038] A highly clean surface of the compound semiconductor substratecan be obtained due to the aforementioned structure, so that anepitaxial n⁺-type ZnSe film having a high carrier concentration can beformed thereon. Thus, it is possible to implement ohmic contact withrespect to a substrate whose carrier concentration can be increased tomerely the latter half of the 10¹⁷ mark.

[0039] In the aforementioned apparatus for fabricating a compoundsemiconductor device according to the present invention, the conductivefilm forming apparatus can form at least one of a Ti film and an Au filmon the compound semiconductor substrate.

[0040] A back electrode implementing ohmic contact with respect to thecompound semiconductor device can be formed due to the aforementionedstructure.

[0041] The foregoing and other objects, features, aspects and advantagesof the present invention will become more apparent from the followingdetailed description of the present invention when taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 is a sectional view schematically showing the structure ofa compound semiconductor device according to an embodiment of thepresent invention;

[0043]FIG. 2 schematically illustrates the structure of an apparatus forfabricating a compound semiconductor device in Example 1 of theembodiment of the present invention;

[0044]FIG. 3 is a sectional view showing the outline of a substrateholder for forming a back electrode of the compound semiconductordevice;

[0045]FIG. 4 is a model diagram showing the state of the surface of ann-type ZnSe substrate subjected to a hydrogen chloride treatment and aradical hydrotreatment;

[0046]FIG. 5 is a sectional structure schematically showing a structurefor measuring the current-voltage characteristic of the back electrodeof the compound semiconductor device;

[0047]FIG. 6 illustrates the current-voltage characteristic of a backelectrode of a ZnSe-based LED according to the present invention;

[0048]FIG. 7 illustrates the current-voltage characteristic of a backelectrode prepared by forming a Ti film and an Au film withoutperforming a radical hydrotreatment;

[0049]FIG. 8 illustrates the current-voltage characteristic of a backelectrode of a ZnSe-based LED according to the present invention;

[0050]FIG. 9 illustrates the current-voltage characteristic of a backelectrode prepared by forming a Ti film and an Au film with atmosphericexposure after forming an n⁺ ZnSe film;

[0051]FIG. 10 is a sectional view showing a stage of forming an LEDepitaxial multilayer film such as an active layer on the surface of ann-type ZnSe substrate in a method of fabricating a compoundsemiconductor device in Example 2 of the embodiment of the presentinvention;

[0052]FIG. 11 is a sectional view showing a stage of bonding the n-typeZnSe substrate of the state shown in FIG. 10 to a glass substrate whilepolishing and etching the back surface;

[0053]FIG. 12 is a sectional view showing a stage of forming an n⁺-typeZnSe epitaxial film on the back surface of the n-type ZnSe substrate inthe state shown in FIG. 11; and

[0054]FIG. 13 is a sectional view showing a stage of forming a Ti filmon the n⁺-type ZnSe epitaxial film in the state shown in FIG. 13.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0055] An embodiment of the present invention is now described withreference to the drawings. Referring to FIG. 1, an n-type ZnSe layer 12,an n-type ZnMgSSe cladding layer 13, a ZnCdSe active layer 14, a p-typeZnMgSSe layer 15, a p-type ZnSe layer 16 and a superlattice layer 17 ofa ZnSe film and a ZnTe film are formed on an n-type ZnSe substrate 1successively from below, and an Au film 18 is provided on thesuperlattice layer 17 as an upper electrode. An n⁺-type ZnSe film 3having a high carrier concentration is formed on the back surface of then-type ZnSe substrate 1 through an interface 2, in order to implementohmic contact. A Ti film 5, which is a conductive layer, and an Au film6 are further formed on the n⁺-type ZnSe film 3 through an interface 4.The Ti film 5 is formed for readily attaining ohmic contact, and the Aufilm 6 is formed for protecting the Ti film 5 readily reacting with theatmosphere. In the aforementioned structure, the interfaces 2 and 4 areparticularly important. According to the present invention, ohmiccontact is attained on these interfaces 2 and 4, for reducinginterfacial resistance.

EXAMPLE 1

[0056] Example 1 of the aforementioned embodiment of the presentinvention is now described.

[0057] 1. Fabrication of Compound Semiconductor Device

[0058] In the present invention, an apparatus for fabricating a compoundsemiconductor device plays an important role. FIG. 2 schematically showsthe structure of the inventive apparatus for fabricating a compoundsemiconductor device. When forming thin films on the surface of ann-type ZnSe substrate, a series of processes of 1) cleaning the surfaceof the n-type ZnSe substrate, 2) forming an n⁺-type ZnSe film having ahigh carrier concentration and 3) forming a conductive layer for anelectrode can be continuously carried out in an ultrahigh vacuum of lessthan 10⁻⁸ Torr. In a gas treatment apparatus 3, a hydrogen chloridetreatment of the ZnSe substrate is employed for the surface cleaningprocess before depositing the tin films, for removing oxygen and carbonfrom the surface of the substrate by etching. When performing thishydrogen chloride treatment at a temperature of not more than 250° C.for protecting an active layer, Cl adsorption results on the surface ofthe compound substrate. Therefore, the aforementioned apparatus movesthe compound semiconductor substrate through an ultrahigh vacuumtransfer path 31 and introduces the same into an MBE (molecular beamepitaxial) apparatus 33, for performing a radical hydrotreatment on thesurface of the substrate subjected to the aforementioned hydrogenchloride treatment with a hydrogen supply source and a radicalizationgun. A Cl adsorption layer is removed by this radical hydrotreatment.Thereafter an n⁺-type ZnSe film having a high carrier concentration isformed in the MBE apparatus 33. Then, the substrate is moved through theultrahigh vacuum transfer path 33 as such, for depositing a Ti layerwhich is a conductive layer and a protective layer therefor in situ in adeposition apparatus 34. Functions of the apparatuses 32, 33 and 34shown in FIG. 2 are described below. It is important that the followingapparatuses 32 to 34 are connected with each other through the ultrahighvacuum transfer path 31.

[0059] (a) The hydrogen chloride treatment apparatus 32 supplies etchinggas of 10 volume % of HCl/balance He from a nozzle.

[0060] (b) The MBE apparatus 33 is provided with Zn, Se, ZnCl₂ and H₂supply sources and the radicalization gun.

[0061] (c) The deposition apparatus 34 includes Au and Ti sources and amask mechanism for depositing Au and Ti.

[0062] An electrode was formed on an n-type ZnSe substrate 1 having acarrier concentration of the latter half of the 10¹⁷ mark prepared byCVT and cleaned with K₂Cr₂O₇/water. As shown in FIG. 3, the n-type ZnSesubstrate 1 subjected to surface cleaning was placed on an Ar foilmember 22 arranged on a substrate holder 21, and a Ta plate 23 coveringits edge portion was pressed with screws 24 for mounting the substrate1. Also when preparing an electrode on a device such as an LED, thesurface of the device is protected with an Au foil member due to thisholding method.

[0063] The prepared Au/Ti/n⁺-type ZnSe/n-type ZnSe substrate 1 wascleaved to not more than 1 mm square, fused to a Cu/W block with In andannealed in N₂ gas at 250° C. for 2 minutes, for evaluating the preparedelectrode. Electric characteristics were measured by current sweepingwith a semiconductor parameter analyzer 4156 by Hewlett-Packard.

[0064] 2. Hydrogen Chloride Treatment and Radical Hydrotreatment

[0065] Cleaning of the surface of the substrate is now described indetail. The features of hydrogen chloride etching reside in thefollowing three points:

[0066] (1) The quantity of etching is larger as compared with radicalhydrogen.

[0067] (2) An extremely smooth surface can be obtained.

[0068] (3) When setting the cleaning temperature (substrate temperature)to not more than 250° C., however, Cl is remarkably adsorbed on thesurface of the ZnSe substrate. Therefore, hydrogen chloride cleaning iscarried out under the following conditions:

[0069] (a) gas composition: 10 volume % of HCl/He balance

[0070] (b) gas pressure: 2×10⁻⁴ Torr

[0071] (c) treatment temperature (substrate temperature): 250° C.

[0072] (d) treatment time: 20 minutes

[0073] With only such a hydrogen chloride treatment, however, noepitaxial film can be obtained also when forming an n⁺-type ZnSe filmlater. This results from the aforementioned highly reactive Cladsorption on the surface of the n-type ZnSe substrate caused by thehydrogen chloride treatment at a temperature of not more than 250° C. Inorder to eliminate this Cl adsorption, a radical hydrotreatment iseffective. The radical hydrotreatment is carried out under the followingconditions, for example:

[0074] (e) H₂ gas flow rate: 1 sccm (standard cubic centimeter/minute)

[0075] (f) H₂ pressure: 1.5×10⁻⁵ Torr

[0076] (g) radical power: 100 W

[0077] (h) radicalization time: 20 minutes

[0078] According to RHEED (reflection high energy electron diffraction)observation, the surface subjected to hydrogen chloride cleaning was(1×1) indicating a crystalline surface, while slight change from (1×1)to (2×2) indicating a clean surface was recognized due to the radicalhydrotreatment. Table 1 shows results of analysis of AES (auger electronspectroscopy) on the surface of the n-type ZnSe substrate after thehydrogen chloride cleaning and the radical hydrotreatment. TABLE 1 SE/ZnZn Se O C Cl Ratio Surface 1) as received 36.6 41.2 3.4 18.5 0.4 1.12 CContaminant and Oxide Present on Surface 2) Hydrogen 41.4 45.4 0.3 2.210.8 1.10 C Contaminant Chloride and Oxide Treatment Reduced, ClAdsorbed 3) Radical 43.7 51.9 0.4 2.2 1.7 1.19 Adsorbed ClHydrotreatment Reduced

[0079] The aforementioned analysis values are relative values (at. %)with reference to Zn, Se, O, C and Cl, and the Se/Zn ratios express atomnumber ratios of Se and Zn. From the results shown in Table 1, a modelillustrated in FIG. 4 can be formed as to the aforementioned cleaning ofthe surface of the ZnSe substrate.

[0080] (1) The surface of the ZnSe substrate is treated with hydrogenchloride for removing oxides and contaminants resulting from carbon fromthe surface by a thickness of about several nm.

[0081] (2) Thereafter the radical hydrotreatment is performed forremoving Cl adsorbed on the surface of the ZnSe substrate over severallayers.

[0082] After performing the aforementioned hydrogen chloride cleaningand the radical hydrotreatment, an n⁺-type ZnSe film was formed and theinterface between the n-type ZnSe substrate and the n⁺-type ZnSe filmwas analyzed by SIMS (secondary ion mass spectrometry). Analysis valuesof impurities were at levels capable of ensuring ohmic contact havingsufficiently low electric resistance.

[0083] 3. Formation of High Carrier Concentration n+ ZnSe Film

[0084] An n+ ZnSe film is formed on the n-type ZnSe substrate subjectedto the aforementioned hydrogen chloride cleaning-radical hydrotreatment.The n+ ZnSe film containing an n-type impurity is formed by MBE bysimultaneously irradiating the substrate with beams of Zn, Se and ZnCl₂.The quantity of doping can be adjusted by controlling a cell temperatureof ZnCl₂. According to this embodiment, an n+ ZnSe film having a highcarrier concentration of the 10¹⁹/cm³ mark can be obtained at atemperature of not more than 250° C. An epitaxial layer having excellentcrystallinity can be formed by setting the ZnCl₂ cell temperature toabout 123° C. and carrying out film formation at 250° C., also whenincreasing the carrier concentration to 5×10¹⁹/cm³. Excellent ohmiccontact can be attained by forming a Ti film on the epitaxial n⁺-typeZnSe film, due to the high carrier concentration of this film. Thethickness of the n⁺-type ZnSe film is preferably set to about 150 to 250nm.

[0085] 4. Formation of Conductive Layer

[0086] Following the aforementioned film formation, the substrate keptin the ultrahigh vacuum is moved in the ultrahigh vacuum transfer pathto be introduced into the deposition apparatus, for forming a Ti film asthe conductive layer and thereafter forming an Au film as the protectivefilm for the Ti film as such. Thus, the substrate can be prevented fromcontamination with oxygen or carbon, dissimilarly to the case of beingexposed to the atmosphere. In other words, the interface 4 shown in FIG.1 can be cleaned for reducing interfacial resistance.

[0087] 5. Current-Voltage Characteristic

[0088] As to a sample shown in FIG. 5 prepared in the aforementionedmanner, the current-voltage characteristic was measured over thevertical direction with the measuring apparatus described in the aboveitem 1. Referring to FIG. 5, the sample is mounted on a Cu/W block 11through an In layer 9. The size of the sample is 1 mm square. FIGS. 6and 7 show influences exerted on the current-voltage characteristic byperformance/non-performance of the radical hydrotreatment after thehydrogen chloride cleaning. Both of inventive and comparative samplesshown in FIGS. 6 and 7 were transferred in ultrahigh vacuum transferpaths after formation of n+ ZnSe films without atmospheric exposure, tobe formed with Ti films and Au films. FIG. 6 shows the current-voltagecharacteristic of the inventive sample of a back electrode prepared byforming an n⁺-type ZnSe film with no Cl adsorption layer, and thereafterforming a Ti film and an Au film. FIG. 7 shows the result of measurementof the comparative sample prepared by carrying out hydrogen chloridecleaning at 250° C. with no radical hydrotreatment, forming an n⁺-typeZnSe film with Cl adsorption, and thereafter forming a Ti film and an Aufilm. As shown in FIG. 6, the inventive sample obtained a substantiallylinear current-voltage characteristic within the sweeping range of −100mA to +100 mA. A voltage generated at 100 mA is 0.21 V. A voltage dropin the substrate and a voltage generated in the In electrode 9 can beestimated as about 0.05 V and about 0.08 V respectively from anothermeasurement, and hence it is estimated that a voltage drop in theelectrode of the aforementioned Au/Ti/n⁺-type ZnSe film is 0.08 V. Inthe comparative sample shown in FIG. 7, on the other hand, Cl adsorbedon the n-type ZnSe substrate influenced on formation of the n⁺-type ZnSefilm, to result in Schottky contact.

[0089]FIGS. 8 and 9 show the current-voltage characteristics of anelectrode prepared by forming an n⁺-type ZnSe film with no Cl adsorptionand thereafter depositing a Ti film and an Au film in an ultrahighvacuum as such and an electrode prepared by forming an n⁺-type ZnSefilm, thereafter exposing the same to the atmosphere and forming a Tifilm and an Au film respectively. The thicknesses of the Ti and Au filmsare 50 nm. It is understood from FIG. 8 that the electrode prepared byforming the films in situ with no atmospheric exposure exhibits a linearcurrent-voltage characteristic, to attain ohmic contact. A voltage dropin the Au/Ti/n⁺-type ZnSe film of this electrode was 0.09 V. In theelectrode prepared by forming a conductive layer after atmosphericexposure, on the other hand, the current-voltage characteristic was notlinear but Schottky contact resulted as shown in FIG. 9.

[0090] As hereinabove described, an electrode of ohmic contact can beformed by employing an n-type ZnSe substrate, performing hydrogenchloride cleaning-radical hydrotreatment on its surface, forming ann⁺-type ZnSe film and thereafter forming a conductive layer withoutatmospheric exposure. The electrode, having extremely low interfacialresistance of not more than 0.1 V, can be employed for the backlight ofa liquid crystal display unit of a portable terminal for contributing toreduction of the number of batteries. Further, such electrodes can besupplied in a large quantity at a low cost without remarkably changingthe conventional fabrication method.

Example 2

[0091] Example 2 of the embodiment of the present invention is nowdescribed. Example 2 is basically similar to the methods of fabricatingcompound semiconductor devices according to the aforementionedembodiment and Example 1 thereof, except some conditions.

[0092] (1) First, an LED epitaxial film including an emission activelayer etc. was formed on the surface of an n-type ZnSe substrate, asshown in FIG. 10. This epitaxial film is identical to the multilayerfilm on the surface of the n-type ZnSe substrate shown in FIG. 1.

[0093] (2) Then, the n-type ZnSe substrate formed with the LED epitaxialfilm was bonded to a glass substrate with wax 25, as shown in FIG. 11.Then, the back surface was back-lapped by a thickness of 250 μm.

[0094] (3) Thereafter etching was performed with a solution ofK₂CrO₇/H₂SO₄, in order to remove distortion introduced by back-lapping(FIG. 11).

[0095] (4) In order to form an epitaxial film on the back surface of then-type ZnSe substrate, the wax 25 was dissolved with acetone fordetaching the ZnSe substrate from the glass substrate. Further,ultrasonic cleaning was performed with acetone.

[0096] (5) The ZnSe substrate was mounted on the substrate holder asshown in FIG. 3, in order to form the epitaxial film on the back surfacethereof. A gold foil member was inserted between the n-type substrateand the substrate holder, for protecting the LED epitaxial film on thesurface side and implementing excellent thermal contact.

[0097] (6) Then, the aforementioned n-type ZnSe substrate was mounted onthe hydrogen chloride treatment apparatus 32 of the treatment apparatusshown in FIG. 2, and hydrogen chloride etching (first cleaning step) wasperformed on the back surface under the following conditions:

[0098] gas pressure: 2×10⁻⁴ Torr

[0099] etching time: 20 minutes

[0100] substrate temperature: 225° C.

[0101] (7) Thereafter the aforementioned n-type ZnSe substrate was movedinto the MBE apparatus 33 through the ultrahigh vacuum transfer path 31in the apparatus shown in FIG. 2. A radical hydrotreatment (secondcleaning step) was carried out on the n-type ZnSe substrate subjected tohydrogen chloride etching with the radicalization gun provided in theMBE apparatus 33. Conditions for the radical hydrotreatment are asfollows:

[0102] hydrogen pressure: 1×10⁻⁵ Torr

[0103] radicalization power: 100 W

[0104] substrate temperature: 225° C.

[0105] treatment time: 20 minutes

[0106] (8) Thereafter an n⁺-type ZnSe film 3 having a high carrierconcentration was epitaxially grown in the MBE apparatus 33 with the MBEapparatus as shown in FIG. 12. In this film forming step, the n-typeZnSe substrate cleaned through the aforementioned first and secondcleaning steps were simultaneously irradiated with beams of Zn, Se andZnCl₂. The conditions for epitaxially growing the n⁺-type ZnSe film 3are as follows:

[0107] substrate temperature: 220° C.

[0108] thickness of n⁺-type ZnSe film: 200 nm

[0109] n-type impurity concentration: 5×10¹⁹/cm³

[0110] (9) Thereafter the aforementioned n-type ZnSe substrate wasintroduced into the deposition apparatus 34 through the ultrahigh vacuumtransfer path 31. As shown in FIG. 13, a Ti film 5 was formed on then⁺-type ZnSe film by 50 nm. Then, an Au film was deposited on this Tifilm 5 by 50 nm, for obtaining an LED similar to that shown in FIG. 1.

[0111] (10) When measuring an operating voltage causing light emissionin the LED fabricated through the aforementioned method, it was possibleto obtain white light from the LED at a low value of 2.6 V. This isbecause it was possible to provide an electrode of ohmic contact havinga low contact potential on the back surface of the n-type ZnSesubstrate.

[0112] When applying this LED to the backlight of a liquid crystaldisplay unit of a portable terminal such as a portable telephone, thenumber of necessary batteries can be reduced due to a low operatingvoltage and hence the commercial value of the portable telephone or thelike can be increased. The aforementioned method of forming an electrodeof ohmic contact on the back surface of an n-type ZnSe substrate, whichis remarkably simple and can be readily carried out in theaforementioned fabrication apparatus, is suitable for mass production.Therefore, the compound semiconductor device can be fabricated in highefficiency with a high yield, to contribute to introduction of alow-priced LED having high performance into the market.

[0113] Although the present invention has been described and illustratedin detail, it is clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims.

1. An apparatus for fabricating a compound semiconductor device,comprising: a gas treatment apparatus performing gas etching on acompound semiconductor substrate; a radical treatment apparatusperforming a radical treatment on said compound semiconductor substrate;a film formation treatment apparatus forming a compound semiconductorfilm on said compound semiconductor substrate; and a conductive filmforming apparatus forming a conductive film on said compoundsemiconductor substrate, said apparatus for fabricating compoundsemiconductor device further comprising an ultrahigh vacuum transferpath coupled with the respective ones of said gas treatment apparatus,said radical treatment apparatus, said film formation treatmentapparatus and said conductive film forming apparatus to be capable oftransferring said compound semiconductor substrate.
 2. The apparatus forfabricating a compound semiconductor device according to claim 1,wherein said gas treatment apparatus includes a hydrogen chloridetreatment apparatus etching said compound semiconductor substrate withhydrogen chloride.
 3. The apparatus for fabricating a compoundsemiconductor device according to claim 1, wherein said radicaltreatment apparatus includes a radical hydrotreatment apparatusperforming a radical hydrotreatment on said compound semiconductorsubstrate.
 4. The apparatus for fabricating a compound semiconductordevice according to claim 1, having an MBE (molecular beam epitaxial)apparatus serving both as said radical hydrotreatment apparatus and saidfilm formation treatment apparatus, wherein said MBE apparatus includesa Zn cell, an Se cell, a ZnCl₂ cell, a hydrogen gas supply source and aradicalization gun for performing a radical hydrotreatment on saidcompound semiconductor substrate with said hydrogen gas supply sourceand said radicalization gas and forming an n-type ZnSe film on saidcompound semiconductor substrate with said Zn cell, said Se cell andsaid ZnCl₂ cell.
 5. The apparatus for fabricating a compoundsemiconductor device according to claim 1, wherein said conductive filmforming apparatus forms at least one of a Ti film and an Au film on saidcompound semiconductor substrate.